Ferroelectric materials are used in a variety of sensor and actuator applications. These may be in the form of piezoelectric or electrostrictive polycrystalline ceramics. Applications for ferroelectric materials include micro-positioning, ultrasonics, stress measurement, active damping, and damage detection.
A ferroelectric material is-one that displays a spontaneous electric polarization. Typically, the material is paraelectric (nonpolar) at high temperatures and becomes ferroelectric below the Curie temperature, at which point it is polarized in a well-defined crystallographic direction. The material may polarize in any one of many crystallographically equivalent directions.
Often a crystal is not uniformly polarized, but rather has multiple xe2x80x9cdomains,xe2x80x9d each of which is polarized in one of the equivalent directions. The spontaneous polarization is accompanied by an elastic strain that depends on the direction of the polarization.
The domain patterns in a single crystal can be reoriented and their boundaries moved by applying an electric field or a mechanical stress to the ferroelectric material. Spontaneous polarization in response to an applied electric field tends to be in a direction closest to a direction of the applied electric field. Spontaneous polarization in response to an applied stress tends to be in the direction in which the strain is best aligned with the applied stress.
The behavior of many ferroelectric materials is characterized by good high frequency response and low hysteresis, however strains in such materials are relatively small, limited to about 0.1%.
According to an embodiment, an apparatus includes a plate of ferroelectric material that is polarized in a first direction. The plate is compressed between opposing faces of the plate. Electrodes contacting each of the opposing faces of the plate are provided for producing an electric field in the ferroelectric material.
At a first voltage, an electric field produced in the plate causes the ferroelectric material to switch polarization to a second direction, producing strain in the plate.
The ferroelectric material may be selected from a number of compounds including, for example, barium titanate, potassium niobate, and lead titanate. The ferroelectric material may also be a solid solution of simple compounds that is ferroelectric at room temperature.
Loading plates can be used to compress the plate. The loading plates may produce a load in a range of from about 1 MPa to about 10 MPa.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.